1. Field of the Invention
The present invention relates to graphitized fine carbon fiber which can be uniformly dispersed in a matrix formed of, for example, resin, ceramic, or metal, and which exhibits high affinity with resin; and to a method for producing the graphitized fine carbon fiber.
More particularly, the present invention relates to graphitized fine carbon fiber which exhibits excellent affinity with resin, dispersibility, and deterioration resistance, and which can impart high surface smoothness to a composite material, the carbon fiber being produced by grinding vapor grown carbon fiber, and then thermally treating the thus-ground carbon fiber at a temperature of at least 2,000° C. in an inert atmosphere; and to a method for producing the graphitized fine carbon fiber.
The present invention also relates to graphitized fine carbon fiber which is useful as a filler material for improving electrical conductivity and thermal conductivity, as an electron emission material for producing field emission displays (FEDs), as a medium for sorption of hydrogen, methane, or various other gasses, and as a material employed in, for example, transparent electrodes, electromagnetic wave shielding materials, and secondary batteries; and to a method for producing the graphitized fine carbon fiber.
The present invention also relates to a battery electrode exhibiting improved charge/discharge capacity and strength, which is produced through incorporation of the graphitized fine carbon fiber into an electrically conductive substrate or through application of the carbon fiber onto the substrate, the battery electrode being employed as a positive or negative electrode of any of a variety of secondary batteries such as dry batteries, Pb storage batteries, capacitors, and recently developed Li-ion secondary batteries.
2. Background Art
Carbon fiber is used in a variety of composite materials, by virtue of its excellent properties such as high strength, high elastic modulus, and high electrical conductivity. In recent years, in conjunction with developments in electronic techniques, carbon fiber has been considered a promising electrically conductive filler for producing electromagnetic wave shielding materials or antistatic materials, and has been viewed as a useful antistatic filler which can be incorporated into resin or as a promising filler employed in transparent electrically conductive resin. Also, by virtue of its excellent tribological characteristics and high wear resistance, carbon fiber has been considered a promising material for use in, for example, electric brushes and variable resistors. In addition, carbon fiber has become of interest as a wiring material for producing devices such as LSIs, since it exhibits high electrical conductivity, thermal conductivity resistance, and electromigration resistance.
Conventional carbon fiber produced through carbonization of organic fiber by means of heat treatment in an inert atmosphere, such as polyacrylonitrile (PAN)-based carbon fiber, pitch-based carbon fiber, or cellulose carbon fiber, has a relatively large diameter; i.e., 5 to 10 μm, and exhibits poor electrical conductivity. Therefore, such carbon fiber has been widely employed as a reinforcement material in, for example, resin or ceramic.
In the 1980's, studies were conducted on a process for producing vapor grown carbon fiber through thermal decomposition of a gas of, for example, hydrocarbon in the presence of a transition metal catalyst. Through such a process, vapor grown carbon fiber having a diameter of about 0.1 to about 0.2 μm (about 100 to about 200 nm) and an aspect ratio of about 10 to about 500 has been produced. A variety of processes for producing vapor grown carbon fiber are disclosed, including a process in which an organic compound such as benzene, serving as a raw material, and an organo-transition metallic compound such as ferrocene, serving as a catalyst, are introduced into a high-temperature reaction furnace together with a carrier gas, to thereby produce vapor grown carbon fiber on a substrate (Japanese Patent Application Laid-Open (kokai) No. 60-27700); a process in which vapor grown carbon fiber is produced in a dispersed state (Japanese Patent Application Laid-Open (kokai) No. 60-54998); and a process in which vapor grown carbon fiber is grown on a reaction furnace wall (Japanese Patent Application Laid-Open (kokai) No. 7-150419).
Since vapor grown carbon fiber is formed of carbon which is readily graphitized, when the carbon fiber is subjected to heat treatment at 2,000° C. or higher, the resultant carbon fiber exhibits excellent crystallinity and improved electrical conductivity. Therefore, the thus-graphitized carbon fiber is employed as an electrically conductive filler material in, for example, a resin or a secondary battery.
A characteristic feature of a vapor grown carbon fiber resides in its shape and crystal structure. The vapor grown carbon fiber has a cylindrical structure including a very thin hollow space in its center portion, and a plurality of graphene sheets (carbon hexagonal network layers) grown around the hollow space so as to form annualar-ring-like tubes. When vapor grown carbon fiber is subjected to heat treatment at 2,000° C. or higher, the cross section of the thus-treated carbon fiber assumes a polygonal shape, and in some cases, micropores are formed in the interior of the fiber.
Since vapor grown carbon fiber has a small diameter, the carbon fiber has a relatively high aspect ratio. Generally, the carbon fibers are entangled with one another to form agglomerates.
Since vapor grown carbon fibers are entangled with one another to form agglomerates, when the carbon fiber is mixed with a matrix formed of, for example, resin or ceramic, the carbon fiber fails to be uniformly dispersed in the matrix, and thus electrical, thermal, and mechanical characteristics of interest cannot be obtained.
In addition, such agglomerated carbon fiber having low bulk density encounters difficulty in kneading with resin. When the surface of the resultant composite material is observed under a scanning electron microscope, the composite material is found to have a “hairy” surface, including pieces of the carbon fiber covered with no resin. When the composite material is employed as an antistatic material for producing, for example, an integrated circuit (IC) tray, due to generation of microscratches at a point at which the tray is in contact with a disk or wafer, the quality of the disk or wafer is lowered, and the yield of a final product is reduced.
Conventionally, various attempts have been made to reduce the length of long carbon fiber through grinding, in order to improve dispersibility of the carbon fiber and to obtain a composite material of smooth surface in relation to the use as a filler, and to produce cut surfaces of the carbon fiber capable of promoting the rate of generation of an intercalate compound in relation to the use of the carbon fiber as a battery material. Conventionally, carbon fiber has been ground through dry grinding by use of, for example, a ball mill, to thereby form short carbon fiber (Japanese Patent Application Laid-Open (kokai) Nos. 1-65144 and 11-322314). However, grinding of carbon fiber through dry grinding involves the following problems. When carbon fiber is ground by use of a ball mill, fine carbon fiber fragments generated through grinding form agglomerates in the mill or the fragments are bonded together, and thus the carbon fiber fails to be micronized efficiently even if grinding is performed for a long period of time. In addition, the resultant carbon fiber fragments have a length as large as about 2 to about 3 μm. When carbon fiber is ground by use of a rod mill, although entangled carbon fibers are fragmented, difficulty is encountered in reducing the length of carbon fiber fragments to 30 μm or less. Meanwhile, wet grinding by use of a bead mill enables highly efficient grinding of carbon fiber. However, wet grinding requires processes subsequent to grinding of carbon fiber, including removal of a dispersant, drying of a solvent, and fragmentation of dried and agglomerated carbon fibers, and thus production cost increases.
When graphite carbon fiber is employed as a grinding raw material (Japanese Patent Application Laid-Open (kokai) Nos. 6-73615, 6-81218, 6-84517, and 11-250911), efficient and uniform grinding of the graphite carbon fiber cannot be performed through dry grinding or wet grinding, since the carbon fiber has high strength attributed to its high crystallinity. In addition, grinding of graphite carbon fiber involves problems, including contamination of the ground carbon fiber with impurities derived from grinding media, and treatment of such impurities.
Since ground carbon fiber has highly active cut surfaces, the carbon fiber tends to interact with a matrix formed of resin, and adhesion between the carbon fiber serving as a filler and the matrix is lowered due to, for example, deterioration of the resin. As a result, the electrical conductivity and thermal conductivity of a composite material are lowered.
Japanese Patent Application Laid-Open (kokai) No. 2002-146634 discloses carbon fiber, each fiber filament of the carbon fiber having a closed end surface. However, this publication does not disclose a carbon fiber filament having a closed end surface and a cut surface.